1. Field of the Invention
The present invention relates to control of high temperature furnaces. More specifically, it relates to the combination of two power supplies to precisely control the temperature of a high temperature furnace.
2. Description of Related Art
High temperature electric furnaces, typically operating in the range of 800 to 2000 degrees Celsius, are used widely in science and industry. One of their common uses is to grow and process the silicon crystals used to make semiconductors. These furnaces have been used to grow materials of interest for laser research. FIG. 1 shows a typical prior art furnace comprising main power supply 2 electrically connected through furnace enclosure 4 to heating element 6 which is located to provide heat to object 8.
In many of the processes, including crystal growth, the quality of the product is strongly affected by the precision with which the furnace temperature can be controlled. Precision is measured in both the dimensions of temperature and time. In many processes, high precision averaged over a long time period is not sufficient. In such processes, the furnace response time is fast and the process is strongly affected by even short term variations in temperature. In the Gradient Freeze method of crystal growth, for example, the location of boundary between the melted feed stock material and the growing crystal is directly affected by the temperature in the furnace. Even a short term variation in furnace temperature can produce a defect in the growing crystal.
Precision control of the furnace temperature requires precise control of the power that drives the furnace. These furnaces typically require tens of kilowatts of electrical power. Power supplies of this capacity are, of economic necessity, somewhat imprecise devices. They are also typically slow to respond to their control inputs, frustrating the use of external instrumentation to improve their quality. At these power levels, power control is almost universally accomplished by control of the raw alternating current (AC) power to the power supply inputs. While such control can be made precise, it cannot be easily made fast at the same time. High precision control of large power supplies has a penalty in quickness of response. At high power levels, it is not practical to have a source of stored energy in the supply that could smooth over transients in the AC prime power. At high power levels, the user is generally forced to share AC mains with other high power users. This can introduce transient power disturbances into the system. The combination of slow control response time in the power supply, coupled with the absence of an energy storage process, means that AC line transients of short duration are passed through the power supply to the load without correction.
Smaller power supplies are radically different in design and commonly have both fast response speed and high precision. If such a supply could be used to add energy to the furnace, its speed and precision could be used to compensate for the imperfections in the main power supply so that the total power delivered would be more precisely controlled. It is generally not possible to combine the outputs of small, fast supplies with that of large, slow supplies. Either series or parallel connections would require that the output stages of the small supply have the power handling capacity of the large supply. A different approach would be to add the contribution of a small, fast supply to that of the large supply in a manner that allowed them to remain electrically separate but summed their powers by independent physical processes in the furnace. The present invention provides such a method.
Further, it is difficult, with the current technology, to make high bandwidth measurements of temperatures in furnaces at high temperatures. In most methods of temperature measurement, the response time of the measurement is dominated by the response time of the temperature measurement probe. In a furnace that had a power control system capable of very quick response, as this invention proposes, it would be necessary to have a means of temperature measurement of equally quick response.
It is desirable to have an apparatus capable of precisely controlling high temperature furnaces. The present invention provides such an article.